CHARGE-TRANSFER AND DISPERSION INTERACTION STABILIZATION OF THE D-2H ISOMER OF O-4

The results of some minimal basis set valence bond calculations, with an antibonding midbond molecular orbital (pi(m)) included, are report ed for the D-2h isomer of O-4. The in-plane pi(m)<--pi* excitations d escribe the charge transfer from each monomer, while the pi<--pi exci tations on each monomer partially describe the intermolecular dispersi ve attractions. It is found that the charge-transfer interactions by t hemselves are insufficient to stabilize the S = 0 spin D-2h dimer of O -4 relative to two O-2 monomers when a correction is included for basi s set superposition error. The inclusion of both the charge transfer a nd dispersion terms yields an estimate of 14 cm(-1) for the binding en ergy (D-e) at an equilibrium separation (R-e) of 3.29 Angstrom. (C) 19 98 John Wiley & Sons, Inc.